Field of the Invention
The present invention relates to methods for joining together objects, and more particularly to brazing methods for joining ceramic objects.
Description of Related Art
Semiconductor processing and similar manufacturing processes typically employ thin film deposition techniques such as Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), Vapor Phase Epitaxy (VPE), Reactive Ion Etching, and other processing methods. In CVD processing, as well as in other manufacturing techniques, a substrate such as a silicon wafer is secured within a processing chamber using semiconductor processing equipment, such as a heater or an electrostatic chuck, and exposed to the particular processing conditions of the process. The heater or electrostatic chuck is essentially a pedestal that, in addition to securing the substrate, can in some instances also be used to heat the substrate.
As heaters are exposed to high operating temperatures and corrosive process gasses, and because good thermal conductivity is required for good temperature control, prior art heaters have been made from a very limited selection of materials, such as aluminum nitride (AlN) ceramic or PBN, silicon dioxide (quartz), graphite, and various metals such as aluminum alloys, nickel alloys, stainless steel alloys, Inconel, etc. Reactive process gasses which are typically used for semiconductor processing, or chamber cleaning, generally react with heaters made with metal alloys. These reactions can produce corrosive by-products and other effects which can be detrimental to the desired process results. Ceramic materials can be much more resistant to reactions with typical process gasses, and to corrosion from reaction by-products. However, ceramic materials can have limited methods of fabrication due to inherent material properties, and have high manufacturing costs.
The manufacture of semiconductor processing equipment using ceramics, such as heaters and electrostatic chucks with a ceramic shaft and a ceramic plate, currently involves hot pressing sub-components to partial density, and then again hot pressing an entire assembly until full density is attained. In this type of manufacture, at least two drawbacks are seen. First, the hot pressing/sintering of a large, complex ceramic piece requires a large physical space, and a multiplicity of sequential sintering steps is required. Second, should a portion of the finished piece become damaged, or fail due to wear, there is no repair method available to disassemble the large piece, likely leading to it being scrapped. In the case of manufacture from two or more pieces which have already been pressed to full density, there are also at least two drawbacks. First, after the initial sintering of the major components, these components are typically joined using a liquid phase sintering process to join the major components (in the case of aluminum nitride, for example), which requires high heat, high compressive force, and a significant amount of time in a process oven capable of providing both the high temperatures and the high compressive force. Often the high compressive force applied to a shaft during this sintering to a plate, such as is done in the process of creating a ceramic heater, requires that the annular shaft walls be of thicker cross-sectional thickness than desired in the finished product in order to support these compressive forces. The shaft may then need to be machined down to a final lesser thickness desired to keep heat flow down the shaft to a minimum. Second, should a portion of the finished piece become damaged, or fail due to wear, there is no repair method available to successfully disassemble a large piece that has been joined in this fashion, likely leading to it being scrapped.
An additional concern may be with regard to the repair of these pieces of semiconductor processing equipment, such as heater and electrostatic chucks with plate and shaft elements. Should a portion of a multi-piece assembly of equipment be damaged, such as due to arcing, for example, it may be desirable to dis-assemble the piece of equipment and re-use portions of it. These portions may retain significant financial value. With current methods of manufacturing, for example with regards to ceramic heaters, there is no method available which would allow for the repair of equipment which would allow replacement of some portions and the re-use of some portions of that equipment.
U.S. Pat. No. 8,789,743 discloses a method for joining ceramic materials which does address the above-mentioned drawbacks of other prior processes. The method includes using a high purity aluminum brazing material at temperatures which result in good and complete wetting, and hermetic joints without diffusion. A limitation on these joints, however, is that the equipment made using these joints cannot be used at higher temperatures, such as 700 C-1400 C, as that is significantly above the solidus temperature of the aluminum braze material.
What is called for is a joining method for joining ceramic pieces which provides a hermetic seal, and which does not diffuse into the ceramics and thus allows for repairs, and which is able to withstand subsequent exposure to processes run at higher temperatures.